Since we investigate new particle formation we need information about the size distribution of particles in the air. Therefore the NAIS (Neutral cluster and Air Ion Spectrometer) measures the size and mobility of aerosol particles and ions in the atmosphere.
The NAIS that we use is manufactured by Airel Ltd., Estonia. With this instrument we are able to measure particles and ions with the size of 2 to 40 nanometers simultaneously.
To measure the ions as well as the particles, the instrument works in two different modes.
Measuring the neutral clusters
First the NAIS splits the air sample into two separate flows to measure positive as well as the negative ions, as can be seen in the figure.
After dividing the sample, the air flow is led to a preconditioning system which neutralises and filters the air sample, and charges it positive and negative, respectively.
The charged particles are now led through the mobility analyser, to measure the mobility diameter. Each size of charged particle leads to a different behaviour in the electric field. The signals are detected by 21 electrometers located in both columns. The mobility diameter can now be converted to a real size of the particles.
Measuring naturally charged ions in the air
To only measure the naturally charged ions in the atmosphere, the preconditioning region is simply deactivated. The air sample is led directly to the mobility analyser, whereas one measures the positively charged ions, and the other one the negatively charged ions.